Concrete is a widely used building material because of its structrual properties, ready availability, and low cost. In many cases, mixed concrete is brought to the job site, poured into forms put up there and allowed to set. However, the use of precast members is becoming increasingly common due to ease and economy in fabrication and assembly. These members are manufactured off the job site and then hauled to the location and erected to form, for example, a wall or deck.
A typical prefabricated member, such as a beam, has a slab or load bearing surface and may include one or more stems containing reinforcing or prestressed strands. To form a building element, the members are positioned so that the edges of the slabs are abutting.
It will be appreciated that relative movement may occur between members forming a portion of a structure. For example, when a deck formed of beam-like members is employed in a building, relative horizontal movement tends to occur between the members in response to wind forces exerted on the vertical walls of the building. Vertical movement between the members may occur by reason of the expansion produced by the sun shining on one portion of a roof deck but not on another or by snow loads existing on one portion of a roof deck but not on another portion.
In order to prevent or lessen such relative movement and to form the individual members into a unitary structure, means may be provided to join the adjacent edges of the members together. Metal pieces may be embedded in the edge of the slab so that when the members are erected, the metal pieces are opposing and may be welded together to provide the necessary joinder. These metal pieces are commonly called weldments.
At the present time, weldments are commonly formed of pieces of the cylindrical reinforcing bars in common use in precast concrete plants. The reinforcing bar is bent in a generally U-shaped configuration. The parallel arms of the bent reinforcing bar are embedded in the concrete and the intermediate web or base is exposed along the edge of the member.
It will be appreciated that the location of the weldment in the concrete of the slab becomes quite critical to the structural strength and properties of the weldment and joinder of the members.
Ideally, the reinforcing bar should be embedded in the middle of the slab in order to insure proper transmission and absorption of the forces received by the weldment.
The intermediate web of the reinforcing bar weldment is usually located on the upper corner of the deck so as to be accessible to a welder stranding on the deck. The parallel portions of a reinforcing bar weldment must therefore slant downward into the slab in order to insure proper embedment and an adequate cover of concrete.
However, during the positioning of the reinforcing bar, once the bar is immersed in the concrete, it thereafter becomes impossible to determine the location of the parallel portions of the bar in the slab. This is particularly true in in view of the fact that the exposed portion of the weldment is cylindrical and thus gives no indication as to whether the remaining poritons of the weldment are properly located. Consistent, proper positioning of the reinforcing bars cannot be assured.
Because of the difficulties in locating reinforcing bar weldments in the slab, the weldments are commonly not embedded sufficiently and pull out of the slab under load, particularly loads applying vertical shear forces to the weldments.
While the arms of the reinforcing bar may be kinked so as to permit the arms to lie parallel to the surface of the slab while positioning the base at the upper corner of the slab, this is often not done or, if done, is not done to the required dimensions so that insufficient anchoring again occurs.
Another shortcoming of reinforcing bar weldments is that the web portion of the reinforcing bar is easily covered with concrete or grout during fabrication of the member. This makes it difficult to find the reinforcing bars in the field.
Even when the reinforcing bar weldments are both embedded and exposed in the proper manner, the welding of the reinforcing bars to join the members together is difficult since the bars are formulated to exhibit the necessary mechanical, reinforcing properties and not for weldability. When weld failure occurs, it is an abrupt brittle failure at an end of the portions heated by the torch or electrode during welding.
Due to the shortcomings of reinforcing bar stock as weldments, specially fabricated weldments have been employed. These have typically comprised a plate having anchors stud welded to the back. While these devices do overcome many of the shortcomings of the reinforcing bar weldments, they suffer other disadvantages. Such weldments tend to be difficult and expensive to fabricate and/or exhibit poor weight to load ratios. They are similarly difficult and expensive to handle and store.